Revert "Remove jl_task_stack_buffer (#54527)" (#54559)

This reverts commit 516ab55.

Unfortunately my PR was premature and broke our CI tests. I'll
get rid of the last remaining `jl_task_stack_buffer` and once
that is done, will re-submit the removal PR.

src/jl_exported_funcs.inc

@@ -454,6 +454,7 @@
     XX(jl_tagged_gensym) \
     XX(jl_take_buffer) \
     XX(jl_task_get_next) \
+    XX(jl_task_stack_buffer) \
     XX(jl_termios_size) \
     XX(jl_test_cpu_feature) \
     XX(jl_threadid) \

src/julia_gcext.h

@@ -135,6 +135,15 @@ JL_DLLEXPORT int jl_gc_conservative_gc_support_enabled(void);
 // NOTE: Only valid to call from within a GC context.
 JL_DLLEXPORT jl_value_t *jl_gc_internal_obj_base_ptr(void *p);
 
+// Return a non-null pointer to the start of the stack area if the task
+// has an associated stack buffer. In that case, *size will also contain
+// the size of that stack buffer upon return. Also, if task is a thread's
+// current task, that thread's id will be stored in *tid; otherwise,
+// *tid will be set to -1.
+//
+// DEPRECATED: use jl_active_task_stack() instead.
+JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *tid);
+
 // Query the active and total stack range for the given task, and set
 // *active_start and *active_end respectively *total_start and *total_end
 // accordingly. The range for the active part is a best-effort approximation

src/task.c

@@ -326,6 +326,34 @@ void JL_NORETURN jl_finish_task(jl_task_t *ct)
     abort();
 }
 
+JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *ptid)
+{
+    size_t off = 0;
+#ifndef _OS_WINDOWS_
+    jl_ptls_t ptls0 = jl_atomic_load_relaxed(&jl_all_tls_states)[0];
+    if (ptls0->root_task == task) {
+        // See jl_init_root_task(). The root task of the main thread
+        // has its buffer enlarged by an artificial 3000000 bytes, but
+        // that means that the start of the buffer usually points to
+        // inaccessible memory. We need to correct for this.
+        off = ROOT_TASK_STACK_ADJUSTMENT;
+    }
+#endif
+    jl_ptls_t ptls2 = task->ptls;
+    *ptid = -1;
+    if (ptls2) {
+        *ptid = jl_atomic_load_relaxed(&task->tid);
+#ifdef COPY_STACKS
+        if (task->copy_stack) {
+            *size = ptls2->stacksize;
+            return (char *)ptls2->stackbase - *size;
+        }
+#endif
+    }
+    *size = task->bufsz - off;
+    return (void *)((char *)task->stkbuf + off);
+}
+
 JL_DLLEXPORT void jl_active_task_stack(jl_task_t *task,
                                        char **active_start, char **active_end,
                                        char **total_start, char **total_end)